Lithium ion rechargeable batteries as a nonaqueous electrolyte battery are widely used in portable electronic devices, such as video cameras, mobile phones, and notebook computers, which have been becoming smaller, lighter, and more powerful. The batteries are also expected to be used in hybrid and electronic vehicles. For such applications, it is recently an important problem to improve the energy density of the batteries.
In order to solve this problem, it is conceivable to increase the maximum charging voltage and the discharge voltage. For lithium ion rechargeable batteries employing LiCoO2 as a cathode active material, the current maximum charging voltage is about 4.2 V vs lithium. At a higher charging voltage of beyond 4.2 V, the amount of deintercalated lithium is excessive, which increases crystal strain of the cathode active material, resulting in collapse of the crystal structure. Thus, it is necessary not only to raise the maximum charging voltage, but also to improve stability of the crystal structure in the charged state, for preventing deterioration of discharge capacity and cycle characteristics.
For improving stability at high voltage of the crystal structure of LiCoO2 as a cathode active material, it is proposed to substitute part of Co with one or more different elements. For example, Patent Publication 1 proposes a cathode active material wherein part of Co is substituted with Mg and M representing at least one element selected from the group consisting of Al, Ti, Sr, Mn, Ni, and Ca. Patent Publication 2 proposes a cathode active material wherein part of Co is substituted with elements of groups IV-A and II-A. Patent Publication 3 proposes a cathode active material that withstands high voltage and has high capacity and excellent cycle characteristics, wherein part of Co is substituted with A representing at least one element selected from the group consisting of Ti, Ta, and Nb, and B representing at least one element selected from the group consisting of Al, Fe, Ni, Y, Zr, W, Mn, In, Sn, and Si. Patent Publication 4 proposes a cathode active material that has high capacity and excellent low-temperature properties, wherein part of Co may be substituted with M representing at least one element selected from the group consisting of Ta, Ti, Nb, Zr, and Hf.
The proposed cathode active materials, however, do not have sufficient stability of LiCoO2 crystal structure. Batteries having high energy density and excellent cycle characteristics are yet to be obtained.    Patent Publication 1: JP-2004-220952-A    Patent Publication 2: JP-2005-50779-A    Patent Publication 3: JP-2001-351624-A    Patent Publication 4: WO-01-027032-A